Optical Properties of Silicon Clusters in the Presence of Water:  A First Principles Theoretical Analysis

• Published in: Journal of the American Chemical Society Vol. 126; no. 42; pp. 13827 - 13837
• Main Authors: Prendergast, David, Grossman, Jeffrey C, Williamson, Andrew J, Fattebert, Jean-Luc, Galli, Giulia
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 27.10.2004
• Subjects: Atomic and molecular clusters; Atomic and molecular physics; Exact sciences and technology; Physics; Reactivity of clusters; Studies of special atoms, molecules and their ions; clusters; Silicon compounds; Semiconductor quantum dots; Electronic structure; Inorganic compounds; Molecular orbital method; Frontier orbital; Molecular clusters; Optical properties; Molecular dynamics method; Theoretical study; Density functional method; Chemical reactivity
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja048038p

We investigate the impact of water on the optical absorption of prototypical silicon clusters. Our clusters contain 5 silicon atoms, tetrahedrally coordinated and passivated with either hydrogen or oxygen. We approach this complex problem by assessing the contributions of three factors:  chemical reactivity, thermal equilibration, and dielectric screening. We find that the silanone (SiO) functional group is not chemically stable in the presence of water and exclude this as a source of significant red shift in absorption in aqueous environments. We perform first principles molecular dynamics simulations of the solvation of a chemically stable, oxygenated silicon cluster with explicit water molecules at 300 K. We find a systematic 0.7 eV red shift in the absorption gap of this cluster, which we attribute to thermally induced fluctuations in the molecular structure. Surprisingly, we find no observable screening impact of the solvent, in contrast with consistent blue shifts observed for similarly sized organic molecules in polar solvents. The predicted red shift is expected to be significantly smaller for larger Si quantum dots produced experimentally, guaranteeing that their vacuum optical properties are preserved even in aqueous environments.